420 Prof. Magnus on the Influence of the Adhesion of 



against it. To give it this temperature, there was in the interior 

 of the space which was heated to 38° C. a narrow brass tube of 

 2 millims. diameter, which, being several times bent, had a length 

 of 4 metres. The air was forced in through this tube, and on 

 its passage became warmed to the same temperature as the pile. 

 If before its entrance into the narrow tube the air had been satu- 

 rated only at 16° C. with aqueous vapours, the pile became 

 heated, and was cooled when the air was dry. This action oc- 

 curred both when the pile was covered with lampblack and when 

 it was perfectly bright and metallic. Condensation of the va- 

 pours took place from air the temperature of which was 22° C. 

 higher than its dew-point, and on a surface which was at the 

 same temperature as this air. 



This effect was very surprising; and I intend to devote myself 

 to a fuller investigation of this deposition of vapours on solid 

 surfaces, which I designate "adhesion of vapour" [Vapor hcesion) . 



As the cooling which occurred when moist air was forced into 

 one of the brass tubes mentioned depends on an altered reflexion 

 on the side of the tube, it might be expected that if reflected 

 heat exclusively reached the pile, this cooling would occur in still 

 higher degree. By means of a plane or concave metal mirror 

 it may be arranged that reflected heat alone reaches the pile. 

 But, however the experiment was modified, no alteration could 

 be observed in the heating effect, whether dry or moist air was 

 forced against the pile. The reason why reflexion from such a 

 mirror acts differently from that from the inside of the tube is, 

 primarily, that the condensation of vapours on the mirror against 

 which the air is blown can never be so complete as in the tube 

 filled with moist air; but this difference depends more especially 

 on the circumstance that there is only one reflexion of a ray from 

 the mirror, while in the tube each ray is repeatedly reflected. 

 Those rays whose direction passes through the axis of the 

 tube undergo but a small number of reflexions : for others the 

 number is more considerable, especially if the angle which the 

 plane of reflexion makes with the plane passing through the axis 

 and the reflecting-point is somewhat large ; for then every ray 

 lying in it is so often reflected that it forms a kind of spiral, 

 which extends over the entire inner surface of the tube. As the 

 diminution of intensity which the rays undergo by reflexion on 

 the surface covered with water increases on repeated reflexions, 

 according to increasing powers of the number of reflexions, it is 

 clear that the water deposited in the tube by the adhesion of va- 

 pour must act quite differently from that on the mirror. When, 

 moreover, the temperature of this latter was only a few degrees 

 lower than that of the air which was forced against it, even on 

 using the mirror there was very definitely a cooling by moist 



